1. Field of the Invention
The present invention relates to a sputtering target and a method of manufacturing the sputtering target. More specifically, the invention relates to a sputtering target of high density and high quality to be used for forming thin films such as electrodes and wiring members of semiconductor devices, and a method of manufacturing the sputtering target.
2. Description of the Related Art
Sputtering has heretofore been used for forming electrodes or wiring in semiconductor devices. Sputtering has been used because it is suitable for mass-production, and it produces high quality films. Sputtering may involve accelerating argon ions to collide against a refractory metal silicide-type target and release the metal silicide, which deposits as a thin film on a substrate adjacent to the target. It is immediately apparent that the properties of the silicide film formed by sputtering strongly depend upon the characteristics of the target.
Due to the increase in the degree of integration and miniaturization of the integrated semiconductor devices, the incidental production of particles (minute grains) from the sputtering target, e.g., during the formation of refractory metal silicide thin films, urgently needs to be reduced. This is because these minute particles (of approximately 0.2 to 10 .mu.m size) which are generated from the target during the sputtering process, tend to be mixed in the deposited thin film, and consequently cause short circuits or the disconnection of wiring when the semiconductor device is used in a circuit, thus reducing the reliability of the device.
Various conventional methods have been proposed for manufacturing targets of high density and fine structure which generate a reduced amount of particles from the target.
For example, Japanese Patent Disclosure TOKKAI-SHO No. 61-179534 discloses a method wherein Si is impregnated into a semi-sintered raw material composed of a refractory metal component (M) and Si component. A structure is obtained having a spherical or oval shaped MSi.sub.2 phase of 5 to 500 .mu.m grain diameter dispersed in "continuous matrix" of Si, with contains impurities such as carbon and oxygen held to less than 50 ppm.
Japanese Patent Disclosure TOKKAI-SHO No. 63-219580 discloses a method wherein a mixture of a refractory metal (M) and Si is subjected to a silicide reaction under a high vacuum thereby forming a semi-sintered substance, which is then subjected to hot isostatic press sintering to obtain a high density target. A compact structure is obtained with a maximum MSi.sub.2 grain size of less than 20 .mu.m and a maximum grain size of free Si held below 50 .mu.m. The target is a mixture of minute MSi.sub.2 grains and free Si grains and contains less than 200 ppm oxygen. Because the oxygen content of the target is low, the resistance of the resultant thin film is also low.
Japanese Patent Disclosure TOKKAI-SHO Nos. 63-179061 and 64-39374 discloses a method wherein a powdered mixture of a refractory metal (M) and Si is subjected to a silicide reaction under high vacuum to obtain a sintered substance. The sintered substance is then pulverized and a composition-adjusting silicide powder is added thereto. The mixture is then subjected to a hot press sintering process to obtain a target having high density and reduced Si coagulation.
When melted Si is impregnated into a semi-sintered substance, it has been found that although a high density target can be obtained, as a result of the substantial reduction of impurities such as carbon, oxygen and the like, the silicon tends to form a matrix. Coarse and large Si portions are formed in large voids in the semi-sintered substance. The Si, having less rigidity than the metal silicide, tends to be broken down by thermal stress during the sputtering process, and because the Si exists in a continuous matrix, the strength of the entire target is reduced. As a result, such metal silicide targets readily collapse, and consequently a large number of particles are produced.
When a semi-sintered substance formed of pulverized Si powder is sintered under applied pressure, it has also been found that although targets of high density and compact structure can be obtained, the carbon contaminant mixed during the pulverizing step remains in the target. The portions of the structure containing excess carbon generate particles during sputtering.
In addition, where pulverized Si powder is formed by sintering under applied pressure into a semi-sintered substance that is not pulverized, a target of high density and dense structure can be obtained.
When pulverized Si powder is formed into a semi-sintered substance by sintering under applied pressure, pulverized, a composition adjusting silicide power is added thereto, and subjected to a hot-press sintering process, a target of high density and fine structure can be obtained. However, carbon contamination of the material increases, as does the amount of oxygen mixed into the material because of the two crushing steps. Accordingly, the amount of particles generated increases, and the electrical resistance of the thin film increases because of the oxygen in the thin film.
Even in the case of a high density target having a relative density of 99%, the amount of generated particles increases due to the impurities, and the products formed when a wiring pattern is etched on the thin film are waste.
Heretofore, because of the ease of controlling the composition of the silicide film, sputtering targets manufactured by a powder-sintering method have ordinarily been used. More specifically, the conventional metal silicide target has been produced by a method wherein a metal silicide (hereinafter noted MSi.sub.2) obtained by reacting metal powder (M) of W, Mo and other metals, with silicon power (Si), and subjecting both to hot pressing or hot isostatic pressing (Japanese Patent Disclosure Nos. 61-141673, 61-141674 and 61-178474).
In such methods, the composition contains MSi.sub.2.2 to MSi.sub.2.9 and the volume of Si phase is 8 to 25 vol. %, which is much less than the volume of the MSi.sub.2 phase. In such a press-sintering operation it is difficult to distribute the Si phase around the angular shaped MSi.sub.2 grains obtained by pulverizing, and accordingly such processes produce a target having a nonuniform structure including coagulated portions of angular MSi.sub.2 grains and localized portions of Si phase.
Another method involves impregnating Si into a semi-sintered silicide (Japanese Patent Disclosure No. 61-58866). In the infiltration process problems are created because the melting point of the MSi.sub.2 phase is much different from the melting point of Si, and depends on the particular metal in the MSi.sub.2 phase. For instance, the melting points of WSi.sub.2, MoSi.sub.2, TiSi.sub.2 and TaSi.sub.2 are 2165.degree. C., 2030.degree. C., 1540.degree. C. and 2200.degree. C., respectively. Where the MSi.sub.2 phase has a melting point differing widely from the melting point of Si (1414.degree. C.), and it is press-sintered at a temperature lower than the eutectic temperature, sintering does not progress between the MSi.sub.2 grains. This reduces the bonding strength between the grains substantially, and creates a brittle product. Furthermore, the residual pores reduce the density of the structure.
When a silicide film is formed by sputtering utilizing such a target, the bonds between the grains tend to be broken by the energy of the argon ions, and particles are generated from the sputtered surface of the target, forming defective portions thereon.
Particularly in a case of high density integrated circuits and the like, particles mixed in the deposited thin film decrease the production of useful devices.
In the latter described conventional method, the composition of the target is controlled by impregnating molten Si into the semi-sintered silicide which has a predetermined density. However, when MSi.sub.2 is synthesized by reacting M powder and Si powder to obtain a semi-sintered substance of a predetermined density, or where a semi-sintered substance of predetermined density is formed by sintering press-formed MSi.sub.2 its density varies depending on the sintering temperature and time, and the applied pressure, so that it is extremely difficult to obtain a target of a desired composition.
According to the knowledge of the present inventors, if the powdered MSi.sub.2 and Si are of high purity, there is no tendency for impurities to diffuse into the boundary between the MSi.sub.2 phase and the Si phase of the target. In the absence of such impurities, the interfacial bonding strengths between the MSi.sub.2 phase and the Si phase, and between the MSi.sub.2 phase regions are high.
In addition, the sputtering operation becomes unstable when there is a large difference between the electrical resistance of the MSi.sub.2 phase and the Si phase. For example, the electrical resistances of WSi.sub.2, MoSi.sub.2, TiSi.sub.2 and TaSi.sub.2 constituting the MSi.sub.2 phase are 70, 100, 16 and 45 .mu..OMEGA..multidot.cm, respectively, while the electrical resistance of the Si phase is 3.times.10.sup.10 .mu..OMEGA..multidot.cm. Furthermore, because there is no interface layer between the MSi.sub.2 phase and the Si phase, the electrical resistance in the boundary region changes abruptly. Particularly in the structure of the target manufactured in accordance with the latter method, high resistance Si phase is directly in contact with the MSi.sub.2 phase of low resistance.
Accordingly, when sputtering is conducted using such target, an insulation breakdown between the MSi.sub.2 phase and the Si phase inevitably occurs, causing electric current to abruptly start to flow. Thus, when the voltage exceeds a predetermined value, discharge of electricity occurs, and MSi.sub.2 grains having weak interfacial bonding strength or parts composed of Si phase are liable to collapse and generate particles.